P2X receptors, which are ligand-gated ion channels, are freely permeable to the three most important physiological cations: sodium, potassium, and calcium. P2X receptors open in the presence of extracellular adenosine triphosphate (ATP) and play an important role in regulating the homeostasis of a range of organisms. P2X receptors are generally classified as ligand-gated cation channels because most of their physiological effects result from the inward flow of cations across the cell surface membrane. The ATP binding site of P2X receptors resides at the interface of neighboring subunits of the multimeric receptor complex.
